This invention concerns an induction cooking hob comprising induction heaters fed by generators.
Induction cooking, or more generally induction heating, uses eddy currents induced in the part to be heated by a high frequency magnetic field, the part being of an electrically conducting material. This part is, for example, a saucepan. The magnetic field is generated by an inductor supplied with a high frequency alternating current by a generator which sets the frequency and amplitude of the current as a function of the heating required. The frequency used for heating depends on a certain number of parameters and in particular the relative magnetic permeability xcexcx of the receptacle and its electrical conductivity "sgr". Starting from the skin thickness, which one takes for example to be equal to half the thickness of the bottom of the receptacle to be heated, one then determined the angular frequency xcfx89 by using the formula:   δ  =            2                        μ          0                ·                  μ          r                ·        σ        ·        ω            
from which one deduces the frequency by the formula:   f  =      ω          2      ⁢      π      
One thus obtains an optimum frequency to be used, of the order of 10 to 50 kHz.
The generator is fed from an electrical supply whose voltage is rectified and filtered. The generator supplied with this rectified voltage U is generally a resonance generator. In effect, the inductors are typically implemented by winding an electrical conductor in a spiral so that, at the operating frequency, the applied load presents this inductor with a resistance R compatible with the power P=U2/R to be transmitted to the load. These same inductors are generally isolated mechanically, electrically and thermally from the load to be heated, which entails an air gap of several millimetres between the load and the inductor. At this distance and in this range of frequencies, the impedance Z=R+jxcfx89L of the loaded inductor is strongly reactive, which entails an inductor quality factor Q=Lxcfx89R greater than  greater than 1. It then suffices to add one or more capacitors C to the inductor, whose inductance is L, to form a circuit resonant at a frequency:   f  =      1          2      ⁢      π      ⁢                        L          ·          C                    
For this reason, the generators are mainly resonance inverters. The impedance Z and in particular the inductance L of the inductor depend on the characteristics of the load. The operating frequencies for a cooking hob with several heaters are in general not identical but close to each other. This phenomenon is on the other hand accentuated by the fact that to retain soft switching modes, power adjustments are generally made by adjusting the operating frequency and therefore two heaters intended to heat identical loads at different powers will use different frequencies. It must be noted that this method of adjustment has the disadvantage of forcing the inverter to operate at frequencies far from its natural resonant frequency, which causes high losses. The best compromise is to have dual thyristor operation while working for maximum power as close as possible to the resonance which is the lowest working frequency and to increase the operating frequency to reduce this power.
These neighbouring frequencies produce beat frequencies which are transmitted to the receptacle being heated and which, owing to their small difference, are in the audible range (a few Hz to a few kHz). These beat frequencies, apart from the noise that they generate in the loads, cause difficulties in the control of independent generators.
To avoid this phenomenon which, because of its amplitude, can make using the product very inconvenient, it is necessary to maintain a good separation between the different generatorxe2x80x94induction heater pairs, which is a major drawback for product modularity; for the same reason, it is for example impossible to heat a large saucepan on several neighbouring heaters fed by different generators.
One known solution consists of supplying neighbouring heaters cyclically for a period varying from a second, for mechanical switching devices, to ten or so milliseconds, for completely electronic solutions. In both cases, the generators must be designed with excess power capacity since power is not transmitted continuously to the heater but is alternating, with a duty cycle which varies according to the power levels demanded from each heater connected to the generator. Furthermore, this cyclic supply can be felt to be a nuisance in use of the device because of the harsh power variations in the load if the period is of the order of a second, or because of the noise related to switching if this period is of the order of a few milliseconds, which corresponds to frequencies of a few hundred hertz.
Another known solution in the field of control and power electronics is to supply the inductor at the same frequency by using generators with hard switching, for example a chopper whose power adjustment method can then be at a fixed frequency in pulse width modulation (PWM) mode. It is however not prudent to use this type of generator to supply standard inductors, in particular because of the high quality-factor of the coils at the operating frequency. In effect this leads to difficulty in making current flow in inductive coils (saw-tooth currents) and major losses when the current in these coils is cut, which requires very large over-capacity in the power generator.
This invention aims to solve these problems and sets out to develop an induction cooking hob having low and high power, and in general, an induction heating device operating at a single frequency or at multiple frequencies to avoid beat frequencies and above all allowing the use of low power generators and in particular modular generators.
To this end, the invention concerns a cooking hob of the type defined above, characterised in that the inductors that are neighbouring or constituting the same heater are fed at the same frequency or at multiple frequencies and in that it includes at least one high power heater comprising at least two inductors having a quasi identical on-load impedance returned to these inductors whatever the load put on this heater. A single controller then governs the generators which operate in resonant mode with soft switching.
Advantageously, this cooking hob includes two induction heaters equipped with inductors, at least one of the heaters (first heater) being high power with at lest two inductors having a quasi identical on-load impedance at maximum load whatever the nature, shape and position of the load placed on this heater. An inverter generator is associated with each heater and operates with soft switching, a single controller governing the two generators. A switching device is associated with the generator of the second heater and has two states:
a normal state for which the switching device connects the generator to the inductor of the second heater,
a power state in which the switching device connects the generator of the second heater to the second inductor of the first heater.
The resonance inverter generators, when they are synchronised in frequency, allow implementation of a high power heater with particularly economical low power generators since they are operating continuously in soft switching mode. A switching device allows the power from two or more generators to be routed to different heaters but it is quite possible to not use this switching device and to permanently connect several generators to one heater, thus increasing its power.
Thanks to the switching device, the cooking hob allows advantage to be taken of the fact that in everyday use of the equipment, it is not necessary for the user to continuously have high power available as much as with induction systems where the power is transmitted directly to the load. The efficiency is particularly high. These power levels are useful during special, short duration preparation sessions (boiling water, heating large quantities of liquid, getting a large grill up to temperature). In continuous use, the power levels needed to maintain cooking (keeping on the boil, simmering) are most often low and can be provided by a single generator.
In this cooking hob, the two heaters can each be formed by two or more inductors having, for each heater, a quasi identical on-load impedance on its inductors; since each heater is associated with a generator, switching devices allow generators of other heaters to be connected to the inductors of the same heater to thus have available high power supplied by several low-power generators and not by a single high-power generator. This allows in particular modular, large-scale fabrication of low-power resonance inverter generators which are moreover usable tin numerous other fields.
The market for power electronics and frequency converters is booming and certain applications are or will shortly be produced by the million for use as frequency converters for control of motors or power supplies intended for microwave oven magnetrons for example. It is thus economically very interesting to be able to benefit from this scale effect, either on the power components or on the control micorprocessors, or on the generators themselves. Major production runs are carried out on low-power converters. The implementation method described allows the use of generators with various power capacities by coupling these low-power converters to heaters in which the power will be equal to the sum of the power capacities of the converters connected to the heater.
The frequency of the different generators connected to a heater must therefore be identical or a multiple of one and the same frequency. The phase of the different generators is in general zero (generators in phase) but it can be advantageous to run the generators in phase opposition in order to accumulate the magnetic flux from neighbouring inductors, which also has the effect of reducing the magnetic field in the immediate vicinity of the inductors. In the case of inductors combined to form the same heater and each having a quasi identical impedance on load, if the same number of combined inductors is in phase and in opposition, then the heater will generate a magnetic field and therefor quasi zero power. By varying the respective phases of the generators connected to the heater from 180xc2x0 to 0xc2x0, it is very easy to achieve variance in the heater power from 0 to the total power of all the generators connected to the heater when they are all in phase. This is particularly interesting since power adjustment can then occur at a fixed frequency which can be chosen to be sufficiently close to the natural resonance of the converter so as to minimise losses in the latter. Power adjustment is much finer since it is difficult to increase the generator operating frequency indefinitely in relation to its natural resonance to reduce the power and, below a certain power, chopping techniques have to be employed to reach sufficiently low power levels. Finally, to cancel out the field of an inductor by controlling the converters connected to it can also be of particular interest with the aim of minimising the leakage magnetic field in the case of inductors not coupled or badly coupled to loads.
According to other advantageous characteristics:
the hob has several low-power generators connected to one or more inductors;
the single controller has a sensor detecting the presence of a load on the inductor to authorise its supply by one or more generators;
the single controller controls the neighbouring inductors so that their electromagnetic fields produce a cumulative flux between the inductors and under the load;
the single controller controls the inductors of the same heater by adjusting the relative phase of the currents supplied by the generators associated with these inductors within a phase shift range between 0xc2x0 and 180xc2x0, in order to adjust the heater power or limit its radiation;
the hob is formed from mass-produced, low-power generators with devices allowing them to be associated and form high-power heaters;
the induction coils are capable of withstanding high temperatures and are arranged as closely as possible to the load while being electrically isolated from it so that the resistance of the coil on load is high considering its inductance;
the generator includes a decoupling capacitor which is split so as to create a quasi fixed capacitive voltage divider;
the switching device comprises a changeover switch to connect the generator of the second heater to the second inductor of the first heater;
the switching device includes a break switch between the junction point of the inductances of the first heater and the capacitors of the resonant circuit of the first inductance of the first heater to close (open) the resonant circuit of the first inductor of the first heater and a changeover switch to connect the inductor of the second heater to its generator or to connect the second generator to the second inductor of the first heater, in series with the first inductor of this first heater and in series with a common capacitor.